Wayne Nash offers his expertise on RC, reverse circulation drilling.
Today, most RC drilling is done either with the direct reverse method or the air-assist method. In the direct reverse method, water or mud flows down the annulus and up the drill pipe. It is lifted by a huge centrifugal pump or a jet eductor. This method works very well with large-diameter holes with water-sensitive sands that are easily contaminated by mud intrusion. As long as the hole is kept full, the hole usually will remain stable. It will not work if the static pressure of the formation is either too low or the well is prone to flow. If the well starts to flow, weight material can be added to the mud to contain the hydrostatic pressure.